Some questions about vacuum tubes

Discussion in 'General Electronics Chat' started by baronobeefdip, Jan 24, 2013.

  1. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    I have been doing some reading on these devices (although obsolete they are still amusing to be plus I feel transistor are much more complicated at this point) and I wanted to ask some questions that I couldn't find through regular google searches.

    From all of the literature that I have been seeing, the input signal is placed in the grid (Duh) and the other end of the signal is placed at the cathode (correct me if i'm wrong). My question is that if the tube is supposed to replicate the original sound signal rather than adding to it (by using the small sound signal to control the bigger current in such a way that it mimics the original signal but larger) I am also aware of the effect each polarity has on the grid, When applied the negative polarity the electrons stop flowing to the plate (or very minimal reach it) but when applied a positive potential the flow of electrons increases (make the voltage more powerful by using the grid to make them flow at a higher velocity).

    I have been getting the impression that the grid is more of an electrically controlled switch and in no way adds more power to the bigger current (the plate voltage connected to the cathode and plate). I thought that the amount of power doesn't increase beyond that of the power supply to the cathode and plate. Am I right assuming that it doesn't produce more voltage than given and instead controls the flow of electrons to the plate rather than using the grid to add more power than what the power supply is providing (the one connected to the plate and cathode). If the grid in fact does add power instead of controlling it without any addition to the power it was supplied with how do I calculate the gain by knowing the applied current (positive make the electrons flow) and the applied power between the cathode and plate?
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    Think of the grid as the handle on a ball valve. The more positive signal on the grid, the higher the plate current. For a more linear amplification, the grid would have a positive bias applied that would insure continued plate current even while the input signal was at its most negative transition. Once upon a time, tube manufacturers published tube guides that provided information about the relationship between grid voltage and plate current. Each tube type will have its own set of curves. To answer your last question, you would need to either find the data for your tube or experimentally develop the chart by varying the grid voltage and monitoring the plate current.
     
  3. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    So the datasheets will contain some type of formula for calculating the amount of amplification is added by using the amount of current that are are applying to the grid and the power (both voltage and current) being attached to the cathode and anode?
     
  4. BillB3857

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  5. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    general, I also want to know why I always see a capacitor and resistor connected in parallel below the cathode in most of the amplifier circuits that are out there?
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    The resistor from the cathode to ground provides a level of self bias and is bypassed with the capacitor in order to provide a more stable bias level. As current flows, a positive voltage will be developed on the cathode which will act as a negative level on the grid. It provides a way, in conjuction with the grid bias, to insure a level of current limiting. It is similar to having a resistor in the emitter lead of a common emitter transistor amplifier.

    Google can be your friend......
    http://en.wikipedia.org/wiki/Cathode_bias
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The resistor is to stabilize the tube bias point by providing local negative feedback for the bias. It reduces the bias variation due to manufacturing differences in transconductance between tubes.

    The capacitor is to bypass the resistor for the AC signal so the AC gain is maximum and not reduced by the negative feedback.
     
  8. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    Why is the cathode always grounded? (perhaps a dumb question but I'm not sure I understand and tube articles are very limited since nobody really uses them)
     
  9. ramancini8

    Member

    Jul 18, 2012
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    You should get a copy of the Receiving Tube Manual put out by RCA. Copies are available on Ebay. If you are going to work on old electronic equipment you need to learn tube theory.
     
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  10. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    Do people even still produce tubes, I'm just wondering and I find transistors to be more complicated (the mechanics not so much but the mathematics and calculations to figure out the amplification factors stump me) unless you think they are easier than tubes.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    The output of a vacuum tube DID NOT replicate its input (but amplified). Instead the vacuum tube was non-linear so it added even-harmonics distortion.

    Simple transistor circuits also add even-harmonics distortion but when many transistors are used in an intergrated circuit they have a very high voltage gain that is reduced with negative feedback. The negative feedback also reduces the distortion so it is extremely small.

    Vacuum tube amplifiers used output transformers that matched the high impedance of the vacuum tubes to the low impedance of a speaker. But the transformers caused phase shift so not much negative feedback could be used so the distortion was fairly high.

    Vacuum tubes slowly burned out and their distortion got worse until they were replaced. Transistors do not burn out unless something else in the circuit fails.

    50 years ago my vacuum tube amplifier needed its output tubes replaced every 3 months. Tubes were cheap then.
    Today many bands replace their vacuum tubes for EVERY show. Tubes are expensive today.
    My transistor amplifier that replaced my vacuum tube amplifier is now 48 years old and it still works perfectly.

    A vacuum tube or transistor can be biased so that its current without a signal is half of maximum. Then it can increase or decrease the current and replicate the input.
    Or it can have a positive and negative supply so that without a signal its output is 0V. Then the signal causes its output to swing positive and negative which replicates the input.
     
    Last edited: Jan 24, 2013
  12. BillB3857

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    Feb 28, 2009
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    Cathodes of vacuum tubes are not always grounded. Cathode followers can be compared to emitter follower transistor circuits. They do not show phase inversion and are frequently used to match impedance. Another circuit that does not ground the cathode is a phase splitter. One output is taken from the plate and the other is taken from the cathode. These two signals are 180 degrees out of phase from each other. It can eliminate the center tapped transformer that is frequently used to drive a push-pull circuit.
     
  13. BillB3857

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  14. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    I thought I give the people on this forum (who happen to be researching vacuum tubes ) a resouce for books on vacuum tubes, this site contains a variety of them.
    http://www.tubebooks.org/
     
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  15. BillB3857

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    Feb 28, 2009
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    Very good. Thanks for posting!
     
  16. baronobeefdip

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    Jan 24, 2013
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  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Heres a supplier of tubes.. yes they are still available.

    http://www.tubesandmore.com/

    Many guitar/bass players still use and love tube amplifiers...even thought they are more costly to maintain.
    One reason is that tubes, even when driven into distortion have a more 'pleasant' sounding distortion. Transistors tend to 'square ' off when driven into clipping, whereas tubes tend to round off when clipping.

    Tube amplifier circuits will have 'horrible' distortion figure according to some audio****'s who will automatically hate them, while others will love them because of the pleasing sound.

    Edit: So there is no confusion I am referring to audiofools, a term used here previously. :)

    I read a few years ago a quote from an engineer who designed state of the art DSP IC chips during the day, After work, he went home and listened to his homebuilt vacuum tube stereo amp. Why ??? because it sounded good.
     
    Last edited: Jan 24, 2013
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  18. baronobeefdip

    Thread Starter New Member

    Jan 24, 2013
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    Thanks, I found that tubes were easier to solder (the sockets that they mount to have bigger soldering points and I feel more precise) as opposed to the transistors which are really tiny and I don't have a magnifying glass on hand (or the money to get a third hand set)

    And that supplier you provided makes the cheapest ones I have found so far, usually I found them at around $80 to $300
     
  19. Audioguru

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    Dec 20, 2007
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    The TubesAndMore website describes the sound of old tubes they sell:
    1) Some are brite and some are not. At ordinary audio frequencies? I don't think so.
    2) Most are noisy and I believe it.
    3) Some have crunchy overdrive? I hope not but they like it.

    I think the guitar players are deaf so they cannot hear all the horrible harmonics their distortion is producing.
     
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  20. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Tubes are large and easy to work with.
    But be VERY VERY carefull... They require dangerous high voltages.

    One rule is 'Keep one hand in your pocket'

    Meaning - don't touch a grounded chassis with one hand while touching another part of the circuit with your other hand.

    I always wear shoes and am standing or sitting with a rubber mat under my feet (no not a rubber room :D) for safety.
     
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